An Internet Explorer vulnerability was patched after being actively exploited in the wild.

According to M86 Security, remote code execution vulnerability CVE-2011-1255, which was patched last month as part of bulletin MS11-050, was being exploited before Microsoft released details about the particular vulnerability.

M86 Security's Avri Schneider said that the company was asked to inspect the URL of a legitimate website of a large private company that was blocked by one of the proactive detection rules implemented in its Secure Web Gateway product.

“After further inspection, we saw that it exploited an unpublished security vulnerability in Internet Explorer. To verify this, we viewed the malicious page on the latest fully patched version of IE and saw a crash followed by execution of malicious code,” he said.

“Based on data we have reviewed from various sources, we can say with a high level of certainty, that the anonymous researcher who (according to Microsoft's security advisory) reported the vulnerability details to VeriSign iDefense, or at least one of his acquaintances, had used the vulnerability details for malicious purposes, as part of targeted attacks.”

The ‘CISSP 2 CISSP' blog said that the compromise looked ‘like the same server used to serve the payload in this attack was used last year to exploit a different zero-day IE vulnerability (CVE-2010-0806)'.

“Because the attacker used an obfuscation method that relies on inserting shell code in the document object model (DOM) via a div element, the M86 researchers were able to use Google to discover over a dozen compromised websites serving this exploit,” it said.

“It is very likely that this vulnerability will be included in drive-by download kits and exploited for some time to come. Symantec has already reported attacks that targeted this vulnerability after Microsoft patched it.

“The malware installed as a result of successful exploitation in Symantec's report is a Trojan that connects to a remote server via HTTP and awaits further instructions.”

Writing three days after the patch was released, Symantec's Joji Hamada said that the only exploits seen affected Internet Explorer 8. “So far, we have only seen limited attacks taking advantage of this vulnerability and believe that the exploit is only being carried out in targeted attacks at present.”

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